A liquid crystal display device of in-plane switching type comprises: a pair of opposed substrates; and a liquid crystal layer interposed between the paired substrates, and therein, a gate line, a source line, a TFT (thin-film transistor), a signal electrode, and a common electrode are disposed on one of the paired substrates (array substrate), whereas a color filter is placed on the other of the paired substrates (opposite substrate).
In this liquid crystal display device, the signal electrodes and the common electrodes are disposed alternately on the same plane, and, through the application of voltage to the signal electrode and the common electrode, an electric field is produced between the signal electrode and the common electrode, so that the orientation of liquid crystal molecules in the liquid crystal layer can be controlled by the electric field. An ever-wider viewing angle can be attained by controlling the orientation of the liquid crystal molecules with this transverse electric field.
In addition, high display quality is required in the recent liquid crystal display device, and thus, high luminance is required in addition to the wide viewing angle, in improving the display quality.
As a technology to achieve both of the above objects, there is a black-matrix-on-array (hereafter referred to as “BOA”) technique (refer to Japanese Unexamined Patent Publication JP-A 2006-301505). According to the BOA technique, a light-shielding film is formed at the side of the array substrate formed with the signal electrode and so forth so as to cover the gate line and the source line, and therefore, in contrast to a case where the light-shielding film is placed on the opposite substrate, there is no need to provide a margin against misalignment between the substrates that may occur during substrate lamination. Accordingly, the width of the light-shielding film can be reduced, and the pixel aperture ratio can be increased, with consequent attainment of ever-higher luminance. Moreover, since the light-shielding film is present on the array substrate formed with the gate line, the source line, the signal electrode, the common electrode, and so forth, it is possible to shield the lines and the electrodes with the light-shielding film in a wide viewing-angle range, and thereby prevent a lowering of contrast caused by reflection and scattering at the line end and the electrode end.
On the other hand, in the liquid crystal display device of a horizontal electric field system, there is a problem that voltage of the signal electrode is easily fluctuated by fluctuation of voltage of the source line, through coupling capacity generated between the source line and the signal electrode, and this influences the display quality.
In order to address this problem, heretofore it has been customary to place a shield electrode in a region between the source line and the signal electrode to alleviate the influence of voltage fluctuations in the source line on the signal electrode.
However, in the case of placing the shield electrode in a region adjacent to the light-shielding film, light from a light source device is blocked by the shield electrode, and consequently the aperture ratio is decreased, which gives rise to the problem of a possible lowering in pixel luminance level. Furthermore, in the case of placing the shield electrode in contact with the light-shielding film in overlapping relation, since the light-shielding film generally does not lend itself to being utilized as a perfect insulator, it follows that a parasitic capacitance between the source line and the shield electrode is increased, which gives rise to the problem of a possible increase in a load to be applied to the source line.
The invention has been devised in view of the problems as mentioned supra, and accordingly an object of the invention is to provide a liquid crystal display device capable of suppressing a decline in pixel aperture ratio and an increase in a load applied to a source line while alleviating the influence of voltage fluctuations in the source line on a signal electrode.